1. Field of the Invention
The present invention relates to a display controller for controlling a liquid crystal display device, a cathode-ray tube (hereinafter referred to as "CRT") display device or the like.
2. Prior Art
A display controller has been proposed which allows a display device such as a liquid crystal display device or a CRT display device to display various images under the control of a central processing unit (hereinafter referred to as "CPU"). Various types of display controllers have been developed and used for a variety of purposes.
In the case where a CRT display device is used as a display device, the display of an image on a screen thereof is conducted in an interlace or a non-interlace method, whereas, in the case of a dot-type liquid crystal display unit being used, the display screen is divided into upper half and lower half display blocks, and the display is conducted by simultaneously scanning both halves.
In either of the above cases, display of a cursor is conducted by, for example, storing dot-data representative of the cursor pattern in a video RAM, and changing the position of the dot-data in the video RAM in accordance with cursor position information. In another known cursor displaying method, a cursor signal is generated such that a cursor in the form of an underline is displayed immediately below the corresponding character displayed on the screen. The first-mentioned method is used mainly in a graphic display mode, while the second-mentioned method is used in a character display mode.
The first-mentioned method relying upon the changing of position of the dot-data in a video RAM, however, suffers from a problem that the CPU has to conduct complicated data processing in order to change the position of the dot-data, whereas, in the second-mentioned method making use of an underline, the shape and display position of the cursor are undesirably limited.
A display controller capable of controlling both of a CRT display device and a liquid crystal display device would be very helpful. However, such a display controller has not yet been developed, because both types of display devices use different scanning methods as explained before. Such a display controller adaptable to both types of display devices, even if such a device is available, would require different methods for designating coordinates of a cursor position for both display devices